Reactions of Metal Ions with Sodium Hydroxide and Ammonia
A concise revision guide to the test-tube reactions of Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺ and Cr³⁺ ions with aqueous sodium hydroxide and aqueous ammonia: observations, ionic equations and what happens in excess.
- 5.3.1(j)(i)-a
- 5.3.1(j)(i)-b
- 5.3.1(j)(i)-c
- 5.3.1(j)(i)-d
- 5.3.1(j)(i)-e
- 5.3.1(j)(ii)-a
- 5.3.1(j)(ii)-b
- 5.3.1(l)-ii
What these spec points say
- 5.3.1(j)(i)-a precipitation reactions, including ionic equations, and the accompanying colour changes of aqueous Cu2+ with aqueous sodium hydroxide and aqueous ammonia
- 5.3.1(j)(i)-b precipitation reactions, including ionic equations, and the accompanying colour changes of aqueous Fe2+ with aqueous sodium hydroxide and aqueous ammonia
- 5.3.1(j)(i)-c precipitation reactions, including ionic equations, and the accompanying colour changes of aqueous Fe3+ with aqueous sodium hydroxide and aqueous ammonia
- 5.3.1(j)(i)-d precipitation reactions, including ionic equations, and the accompanying colour changes of aqueous Mn2+ with aqueous sodium hydroxide and aqueous ammonia
- 5.3.1(j)(i)-e precipitation reactions, including ionic equations, and the accompanying colour changes of aqueous Cr3+ with aqueous sodium hydroxide and aqueous ammonia
- 5.3.1(j)(ii)-a complex formation, including ionic equations and the accompanying colour changes, with excess aqueous sodium hydroxide
- 5.3.1(j)(ii)-b complex formation, including ionic equations and the accompanying colour changes, with excess aqueous ammonia
- 5.3.1(l)-ii interpretation and prediction of unfamiliar reactions including precipitation
From Aqua Ion to Hydroxide
Choose a step to watch hydroxide ions take protons from the water ligands of a metal aqua ion until a neutral complex precipitates.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
Before You Start
This page builds on Ligands, Complex Ions and Their Shapes and Ligand substitution and the Stability of Complexes.
What you need to remember
- A precipitate is an insoluble solid that forms when two solutions are mixed.
- Aqueous sodium hydroxide gives a blue precipitate with copper(II) ions: this test was met before A Level.
- An aqueous metal ion is written here as Cu²⁺(aq). It is really an aqua ion such as [Cu(H₂O)₆]²⁺.
- Aqueous ammonia is a weak base: NH₃(aq) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).
- Ammonia is also a ligand: it has a lone pair on the nitrogen atom.
GCSE Recap: Testing for Ions
Three quick questions on the GCSE ion tests this page builds on.
The Observations
Each test starts with a few drops of the reagent, which is then added until it is in excess.
| Ion | Colour of the aqueous ion | A few drops of NaOH(aq) or NH₃(aq) | Excess NaOH(aq) | Excess NH₃(aq) |
|---|---|---|---|---|
| Cu²⁺ | blue solution | pale blue precipitate, Cu(OH)₂ | no further change: precipitate remains | dissolves: deep blue solution, [Cu(NH₃)₄(H₂O)₂]²⁺ |
| Fe²⁺ | pale green solution | green precipitate, Fe(OH)₂; turns orange-brown at the surface on standing in air | no further change: precipitate remains | no further change: precipitate remains |
| Fe³⁺ | yellow-brown solution | orange-brown precipitate, Fe(OH)₃ | no further change: precipitate remains | no further change: precipitate remains |
| Mn²⁺ | very pale pink solution (almost colourless) | off-white (pale brown) precipitate, Mn(OH)₂; darkens to brown on standing in air | no further change: precipitate remains | no further change: precipitate remains |
| Cr³⁺ | green solution (violet when pure) | grey-green precipitate, Cr(OH)₃ | dissolves: dark green solution, [Cr(OH)₆]³⁻ | dissolves: purple solution, formed slowly, [Cr(NH₃)₆]³⁺ |
- A few drops of either reagent give the same precipitate: the metal hydroxide.
- The differences appear only when the reagent is in excess.
- Pure aqueous Cr³⁺ is violet, but solutions of chromium(III) salts usually look green.
Copper(II) ions: a pale blue precipitate with a few drops of either alkali, which dissolves only in excess ammonia to give a deep blue solution.
A Few Drops: Precipitation of the Hydroxide
Hydroxide ions react with the aqueous metal ion to form an insoluble metal hydroxide. One OH⁻ ion is needed for each positive charge on the metal ion.
| Ion | Ionic equation | Observation |
|---|---|---|
| Cu²⁺ | Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) | blue solution → pale blue precipitate |
| Fe²⁺ | Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s) | pale green solution → green precipitate |
| Fe³⁺ | Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) | yellow-brown solution → orange-brown precipitate |
| Mn²⁺ | Mn²⁺(aq) + 2OH⁻(aq) → Mn(OH)₂(s) | very pale pink solution (almost colourless) → off-white (pale brown) precipitate |
| Cr³⁺ | Cr³⁺(aq) + 3OH⁻(aq) → Cr(OH)₃(s) | green solution (violet when pure) → grey-green precipitate |
- Aqueous ammonia gives the same precipitates, because it contains OH⁻ ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻.
- The ionic equation is therefore the same for both reagents.
- Include state symbols: the (s) shows the precipitate.
Changes on standing
- The green iron(II) hydroxide turns orange-brown at the surface: oxygen in the air oxidises it to iron(III) hydroxide.
- The off-white manganese(II) hydroxide darkens to brown as air oxidises it.
Iron(II) ions give a green precipitate and iron(III) ions an orange-brown precipitate. Neither dissolves in excess sodium hydroxide or ammonia.
Manganese(II) ions give an off-white precipitate that darkens in air and does not dissolve in excess of either reagent.
Check: Hydroxide Precipitates
Use the idea that the precipitate is the neutral metal hydroxide: count the hydroxide ions needed, write ionic equations and explain why both reagents give the same solid.
Excess Sodium Hydroxide: Amphoteric Hydroxides
An amphoteric hydroxide reacts with both acids and bases. It dissolves in excess sodium hydroxide and also in acid.
| Hydroxide | With excess OH⁻ (acting as an acid) | With H⁺ (acting as a base) | Observation in excess NaOH |
|---|---|---|---|
| chromium(III) hydroxide | Cr(OH)₃(s) + 3OH⁻(aq) → [Cr(OH)₆]³⁻(aq) | Cr(OH)₃(s) + 3H⁺(aq) → Cr³⁺(aq) + 3H₂O(l) | grey-green precipitate dissolves: dark green solution |
- In excess sodium hydroxide a complex ion forms, so the precipitate dissolves.
- The hydroxides of Cu²⁺, Fe²⁺, Fe³⁺ and Mn²⁺ do not dissolve in excess sodium hydroxide. They are basic, not amphoteric.
- Every hydroxide on this page dissolves in dilute acid to give back the aqueous metal ion.
Chromium(III) hydroxide dissolves in excess sodium hydroxide to give a dark green solution and in excess ammonia to give a purple solution.
Key idea: Amphoteric means able to react with both acids and bases. A hydroxide that dissolves in excess sodium hydroxide and in acid is amphoteric.
Excess Ammonia: Ligand Substitution
In excess, ammonia acts as a ligand. Some hydroxide precipitates dissolve because ammonia molecules replace ligands round the metal ion.
| Ion | Equation | Observation |
|---|---|---|
| Cu²⁺ | Cu(OH)₂(s) + 4NH₃(aq) + 2H₂O(l) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2OH⁻(aq) | pale blue precipitate dissolves: deep blue solution |
| Cr³⁺ | Cr(OH)₃(s) + 6NH₃(aq) → [Cr(NH₃)₆]³⁺(aq) + 3OH⁻(aq) | grey-green precipitate dissolves: purple solution, formed slowly |
- This is ligand substitution: the complex ion formed is soluble, so the precipitate dissolves.
- With copper the substitution is incomplete: only four ammonia ligands enter, and two water ligands remain.
- The hydroxides of Fe²⁺, Fe³⁺ and Mn²⁺ do not dissolve in excess ammonia.
- The overall ligand substitution equations are on Ligand substitution and the Stability of Complexes.
Remember: A few drops of ammonia: it is a base, and the hydroxide precipitates. Excess ammonia: it is a ligand, and some precipitates dissolve as complex ions.
Check: Excess Sodium Hydroxide and Excess Ammonia
Explain what dissolving in excess shows: amphoteric behaviour with sodium hydroxide, and ligand substitution with ammonia.
Using the Tests to Identify Ions
- Fe²⁺ gives a green precipitate and Fe³⁺ an orange-brown precipitate. Neither dissolves in excess of either reagent.
- Cr³⁺ and Fe²⁺ both give green precipitates. Only the chromium(III) hydroxide dissolves in excess sodium hydroxide.
- Mn²⁺ gives an off-white precipitate that darkens in air and dissolves in neither reagent.
- Cu²⁺ is recognised by the deep blue solution in excess ammonia.
Worked example 1
A green solution gives a grey-green precipitate with a few drops of sodium hydroxide. The precipitate dissolves in excess to give a dark green solution. Identify the metal ion and write the equations.
Step 2. Precipitation: Cr³⁺(aq) + 3OH⁻(aq) → Cr(OH)₃(s)
Step 3. In excess: Cr(OH)₃(s) + 3OH⁻(aq) → [Cr(OH)₆]³⁻(aq)
Answer. The ion is Cr³⁺, and its hydroxide is amphoteric.
Worked example 2: an unfamiliar ion
An ion X²⁺ forms a hydroxide precipitate with aqueous sodium hydroxide. Write the ionic equation.
Answer. X²⁺(aq) + 2OH⁻(aq) → X(OH)₂(s)
These test-tube reactions are part of practical work: see Practical Activity Groups.
Quick Check: Separating a Mixture of Ions
Use the data in the question to follow each metal through two filtrations.
Common Exam Points
Say
- “A pale blue precipitate forms, which dissolves in excess ammonia to give a deep blue solution.”
- “Chromium(III) hydroxide is amphoteric: it dissolves in excess sodium hydroxide and in acid.”
- “Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)”, with state symbols.
Do not say
- “The solution turns cloudy” (state the colour and that it is a precipitate).
- “A blue solution forms” for copper(II) hydroxide (it is a pale blue precipitate).
- “Cu²⁺ + 2NH₃ → Cu(NH₃)₂” (a few drops of ammonia give the hydroxide).
Watch for
- Observations need a colour and a state: solution or precipitate.
- An equation for a reaction in excess starts from the hydroxide precipitate.
- Charges must balance: check the total charge on each side.
- Iron(II) hydroxide darkening in air is oxidation, not a reaction with the reagent.
Check: Identifying Ions from Test-Tube Reactions
Interpret described observations with sodium hydroxide and ammonia, write the ionic equations and decide which reagent tells two ions apart.
FAQs
Use these quick answers to check the reactions of aqueous metal ions with sodium hydroxide and ammonia.
What do I see when sodium hydroxide is added to copper(II) ions?
A pale blue precipitate of copper(II) hydroxide forms: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s). The precipitate has no overall charge, which is why it is insoluble. It does not dissolve in excess sodium hydroxide.
Why does a little ammonia give the same precipitate as sodium hydroxide?
Aqueous ammonia is a weak base, so it contains hydroxide ions: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. A few drops therefore precipitate the same metal hydroxide, such as pale blue Cu(OH)₂. Only excess ammonia acts as a ligand.
Why does the copper(II) hydroxide precipitate dissolve in excess ammonia?
Ammonia molecules act as ligands and form a soluble complex ion by ligand substitution: Cu(OH)₂ + 4NH₃ + 2H₂O → [Cu(NH₃)₄(H₂O)₂]²⁺ + 2OH⁻. The pale blue precipitate dissolves to give a deep blue solution.
How do I tell iron(II) ions from iron(III) ions?
Add sodium hydroxide solution. Iron(II) ions give a green precipitate, which turns orange-brown at the surface as oxygen in the air oxidises it. Iron(III) ions give an orange-brown precipitate straight away. Neither precipitate dissolves in excess.
What does amphoteric mean?
An amphoteric hydroxide reacts with both acids and bases. Grey-green chromium(III) hydroxide dissolves in excess sodium hydroxide to give a dark green solution of [Cr(OH)₆]³⁻. It also dissolves in acid, giving Cr³⁺(aq) again.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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